Optical fiber waveguides, which are able to amplify light have been explored over the last decades for a number of applications (see e.g. Michel J. F. Digonnet, ed., “Rare-Earth-Doped Fiber Lasers and Amplifiers”, 2nd edition, 2001, Marcel Dekker, Inc., New York-Basel, referred to elsewhere in this application as [Digonnet]). For optical communication systems, for example, fiber optical amplifiers are used, where Erbium ions are incorporated into an optical fiber to provide amplification of light around 1.5 μm. For amplification at other wavelengths other rare-earths, such as Yb, Nd, Ho, Tm, and others are used. Other means of amplification than using rare-earth ions are also possible, for example by filling active material in voids of so-called photonic crystal fibers (also known as micro-structured fibers, holey fibers, hole-assisted fibers, photonic bandgap fibers), see e.g. Bjarklev, Broeng, and Bjarklev in “Photonic crystal fibres”, Kluwer Academic Press, 2003 (referred to elsewhere in this application as [Bjarklev et al.]) for a general introduction to the design, manufacturing and properties of these fibers. In general, there is a need to control the spectrum of amplified light in an optical amplifier. In this application, the invention is exemplified using rare-earth doped optical fibers, but the concepts and ideas also cover other types of amplifying optical waveguides.
In a fiber amplifier, active ions are pumped by an optical source to an excited energy level and through stimulated emission amplification takes place. The amplified wavelength(s) can be controlled through an input (or seed) signal and/or it can be controlled by feedback mechanisms such as wavelength selective mirrors in a laser cavity. However, the available wavelengths for amplification are limited by the available emission spectrum of the specific rare-earth ions. The emission spectrum of a given rare-earth ion depends to some degree on the exact host material that it is incorporated into—and over the past decade significant resources has been spent on investigating various rare-earths and host compositions to provide desired emission spectra (and absorption spectra to suit desired pump sources), cf. e.g. [Digonnet], chapter 2.
One common problem for optical amplifiers is that it is difficult to obtain amplification for parts of the emission spectrum, where the emission cross-sections of rare-earth ions are significantly below their peak values. The problem is that amplified spontaneous emission at undesired wavelengths (where the emission cross-sections are high) can dominate over the stimulated emission at a desired (e.g. signal) wavelength (with a lower emission cross-section). For example, for Yb doped fibers it is in practice difficult to obtain amplification at wavelengths above 1100 nm, and in particular for wavelengths above 1200 nm, as amplified spontaneous emission in the wavelength range around 1030 nm-1070 nm builds up and de-excite the Yb ions (cf. e.g. FIG. 4). Another example is Yb doped fiber amplifiers that are desired for amplification around 980 nm, where amplified spontaneous emission around 1030 nm-1070 nm also plays a limiting factor. Another example is ErYb doped fibers, where desired amplification around 1.5 μm is limited by amplified spontaneous emission from the Yb ions. In general, the amplified spontaneous emission can develop into lasing at undesired wavelength(s) in fiber configurations with optical feedback mechanisms.
A typical solution to suppress undesired amplified spontaneous emission in optical amplifiers is to divide the optical fiber amplifier into a number of amplifier stages, where optical filters, which filter out amplified spontaneous emission, are inserted between the amplifier stages. It is, however, a disadvantage that multiple optical components are required to filter away the undesired amplified spontaneous emission. Further, there are in practice limits to power levels and wavelengths that can be obtained in this way. These practical limits are governed by filtering efficiency and differences in emission cross-sections at undesired and desired wavelengths.    [Argyros et al.] (Argyros et al. in Optics Express, Vol. 13, No. 7, 4 Apr. 2005, pp. 2503-2511) describe guidance properties of low-contrast, passive PBG fibers.
WO-03/019257 describes an optical fiber comprising a core (e.g. a low-index feature, e.g. a void) an outer air-clad layer for providing a high NA and a number of periodically distributed cladding features in an inner cladding to provide light guidance due to the PBG-effect. In an embodiment, the optical fiber comprises an optically active material whereby the optical fiber may be used for optical amplification or for lasing. The PBG guidance may be used to enhance specific parts of the amplifier spectrum by placing a bandgap edge at a frequency within the emission spectrum of the active ion (cf. FIG. 28 in WO-03/019257).    [Bouwmans et al.] (Bouwmans et al. in Optics Express, Vol. 13, No. 21, 17 Oct. 2005, pp. 8452-8459) describe a solid core photonic bandgap fiber.